RTX 4090 BF16 unlock via UEFI toolbench: ~10% faster image gen, still trails RTX 5090
devmoching · reddit · 2026-10-31
A Reddit user unlocked BF16 on an RTX 4090 under Windows 11 using MarvinFS's rtx4090-uefi tool (a UEFI app that unlocks FP16/BF16 FP32-accumulate tensor throughput), then benchmarked it against an RTX 5090 in ComfyUI on identical hardware (Ryzen 7 9800X3D, 192GB DDR5, same driver, --use-sage-attention --fast).
Key results:
- 10% average speedup on BF16 image/video generation; LoRA training (AI-Toolkit, 1000 steps) improved 8%
- Single image: 5090 23.15s vs unlocked 4090 27.62s vs stock 30.11s
- Why gains are modest: modern MMDiT models are too large — BF16 weights don't fully fit in 24GB VRAM, and offloaded weights bottleneck on PCIe Gen4 x16 bandwidth, so the big GEMM throughput gains can't be fully realized
Verdict: worth trying on an out-of-warranty 4090, but expect incremental gains, not a 5090 killer.
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